High-power multipactor phase shifter



April 1968 w. G. SPAULDING 3,377,573

HIGH-POWER MULTIPACTOR PHASE SHIFTER 2 Sheets-Sheet 1 Filed March 2, 1966 DC BIAS SUPPLY William G. Spaulding, INVENTOR. N 721. M BY M J. Ml? W M M e. M

April 1968 w. G. SPAULDING 3,377,573

HIGH-POWER MULTIPACTOR PHASE SHIFTER Filed March 2, 1966 2 Sheets-Sheet 2.

William G. Spaulding,

INVENTOR.

BYM 1% WM Maw United States Patent 3,377,573 HIGH-POWER MULTIPACTOR PHASE SHIFTER William G. Spaulding, Huntsville, Ala., assignor to the United States of America as represented by the Secretary of the Army Filed Mar. 2, 1966, Ser. No. 533,128 Claims. (Cl. 333-98) This invention relates in general to high-power phase shifters and more particularly to phase shifters utilizing a variable density multipactor discharge for controlling the propagation constant of a microwave transmission line.

The phased-array antenna is gaining considerable interest in modern weapons systems because of its inertialess, high-speed scanning rate and multifunction capabilities. Its use permits a conceivable system that can engage a dozen or more simultaneous targets, track them, steer the intercept missiles and search for additional threats all in the same operational cycle. The phased array consists of a large number of individual radiating elements, each of which radiates microwave energy in proper phase with respect to the others to allow the sum to combine in space in a particular direction. Thus, a plane wave front is produced which is steerable by the electrical phase of the elemental radiators.

To obtain azimuth and elevation steering, separate phase intelligence must be programmed to the radiating elements for each. This may be done by phase-phase scanned arrays or by frequency phase scanned by rows and columns or by subarrays.

The frequency-phase scanning generally employs a phased higher power signal at the input of each frequency scanned series of low power radiating elements. If postamplification phasing is employed a high power phase shifter is required before the power is divided to the elemental level on all of these configurations. If preamplification phasing is employed, the phase shifter is a lower power device but more power amplifier tubes are required.

Another use for a high power phase shifter is in applications where it is necessary to combine two or more high power microwave signals. For success, it is required that the signals be in phase. It is apparent that a high power phase shifter can be used to fulfill this requirement.

As previously stated, control of the phase of a very high power microwave signal is one of the basic requirements to be considered in the development of a high power, phased-array transmitter. The state-of-the-art devices for this purpose generally employ ferrites. The threshold characteristics of the materials used in these devices impose inherent limitations at high peak and average energy levels. In addition, the devices are sensitive to temperature change, hysteresis effects, and present many problems concerned with minimizing their control power, their time constant, and the effects of their stray magnetic fields.

Varactor diodes have been used in electronically controlled phase shifters. A multiplicity of the diodes is required in high power applications. To distribute the energy equally among them appears to be a formidable problem in a practical device.

The use of ionized gases within a transmission line to control the phase shifter has been investigated by several workers using many configurations. These devices depend on the change in dielectric constant that occurs in a gas discharge as the density of free electrons is varied. This effect is due to the interaction of the free electrons with the electromagnetic fields. In the above devices, there is an inherent signal power limitation, be-

Patented Apr. 9, 1968 cause at the higher power levels the gas is ionized by the signal itself and control of the electron density is lost. If the electron density becomes excessive the power absorbed and/ or reflected by the discharge is prohibitive. In addition, there are disadvantages associated with their relatively high gas pressures due to the frequency of collision of the free electrons with the heavier ions and new tral molecules. This represents an insertion loss. Ionization and deionization times also create problems when used with pulsed signals.

The present invention overcomes the disadvantages of the above mentioned prior art phase shifters by utilizing the phenomenon known as multipactor discharge, which will be discussed in detail hereinbelow, for controlling the effective dielectric constant of a microwave transmission line.

The multipactor phase shifter is of the general type as the devices mentioned above because it also depends on the interaction of microwave energy with a controlled density of free electrons; however, the difference and the advantages of the disclosed phase-shifter lie in the mechanism by which the free electrons are obtained and the pseudo-coherent relationship between the phase of the signal and the velocity vector of the free electrons. Also, the phase shifter is a vacuum device eliminating the problems caused by the relatively higher pressure gas discharges.

The characteristics for which this invention shows promise for superiority over existing devices are listed below:

(1) Power Operation at very high peak powers is possible because the limitation is simply the threshold of field emission or are discharge of the transmission line across a vacuum dielectric. A phase shifter must be designed to operate over a specific range of power level. For instance, at C band, it is possible for one to choose a center, peakpower level of approximately 50 kw. to 5 megawatts. The device would operate through a limited power range about the center, power level chosen. The limits of this power range are determined by the phase shift accuracy required. Present state of the art ferrite phase shifters are limited to about kw. peak power as a material threshold maximum at C band.

High average powers may also be attained because the heat due to electron collision at the secondary emission surfaces is distributed over a large area. This area can be readily cooled by air or water because it is metal and forms part of the vacuum envelope. Other present devices must be cooled by conducting the heat out of materials which have a low thermal conductivity in comparison with the metallic surfaces used here.

(2) Temperature efiects Secondary emission is not highly dependent on temperature, and primary emission temperatures of any consequence are not encountered. Present ferrite device technology is struggling with temperature compensation problems due to basic materials phenomena.

(3) Form factor The ultimate form factor for this device would appear to be a length of transmission line no larger in cross section than a standard a=2b waveguide and no longer than about six to eight guide wavelengths. This is comparable to present ferrite devices of much lower power handling capability. This size of the multipactor device does not change its higher power requirements whereas the size of other devices using present techniques must be increased significantly for higher power design.

(4) Response time The two-surface multipactor phase shifter, using a form of voltage control, has a very fast response. No appreciable inertia is involved except in the small capacitance between the two surfaces and the control grid, if used. The single surface multipactor, using magnetic field control, will have the inertia always involved in producing and changing a strong magnetic field.

The multipactor discharge builds up exponentially very rapidly. If n chance electrons are accelerated by the first half cycle of the RF, and if they release 6 secondaries each, where 6 is the secondary emission coefficient, then after m half cycles have passed, the total number of electrons (n) is One chance electron impinging on a material with a secondary emission coefiicient of would release (five) electrons after ten RF cycles (two nanoseconds at five -kmc.). This order of buildup time should be negligible for most applications. Cutoff or decay time is, for all practical purposes, instantaneous if the trailing edge of the RF pulse is instantaneous.

Thus, the rise time and cutoff time is much faster than previously proposed gas filled devices. The response to voltage control shows promise of being considerably faster than analogue ferrite devices and comparable to latching type ferrite devices.

In accordance with the above, it is an object of this invention to provide a means for electronically controlling the phase of very high power microwave signals.

The simplest form of multipactor, occurs between two surfaces of a certain con-figuration in a vacuum when an RF voltage is impressed across them. Initial stray electrons in the RF field are accelerated to one of the surfaces, depending on the instantaneous polarity. Upon collision at the surface, these primary electrons release secondary electrons which are also accelerated by the RF field. If the distance between the two surfaces is such that the electron transit time from one surface to the other is equal to one-half the period of the RF, and if the secondary emission coefficient is greater than unity, a secondary electron resonance occurs. This resonance is commonly called multipactor, and it consists, therefore, of a secondary electron cloud traveling back and forth between the two surfaces in synchronism with the RF voltage.

Multipactor action can be sustained at one surface if a DO. bias voltage is impressed across the RF gap. This provides a restoring force enabling the electrons to return to the surface of emission.

Single surface multipactor action can also be achleved by applying a magnetic field between the su rfaces which provides the restoring force due to the v b component of the Lorentz force equation. This action can also be obtained by using both a DC. bias and a magnetic field.

We see then that the required conditions for multipactor are: (1) surface materials which are good secondary electron emitters, (2) a vacuum envelope, and (3) an electron transit time equal to one-half (or an odd integer times one-half) of the RF period for the single-surface type, or an integer times the RF period for the two-surface variety. The latter condition implies that the distance of travel, the frequency, and the magnitude of the applied signal have a unique relationship. In actuality, this relationship takes place over a range of values of these parameters.

It is also important to realize that the source of energy which sustains the secondary electron resonance is the applied RF signal. In a practical device, this'energy must be a small fraction of the total power to be passed, and it represents an insertion loss. When such a discharge is uncontrolled, it is limited by the space charge of the abundant negatively-charged electrons and/or the loading of the RF voltage. If the insertion loss is to remain satisfactory, the practical phase shifter must have a means of controlling the density of electrons in the discharge in a defineable manner and without approaching the space charge/RF loading limitation.

An abundance of work has been done on the propagation of microwave energy through plasma. It has been applied by workers interested in the ionosphere, evolving microwave devices, in the measurement of the properties of plasmas, and recently in reentry problems. These studies have shown that the mechanism of interaction between plasma and electromagnetic energy is the force exerted on the free electrons by the electric fields. This creates an effective, complex, dielectric constant which depends on the density of the free electrons. The interaction of RF and the electrons of the multipactor discharge is based on the same theoreticalequations used in the study of plasmas. Therefore, the effective dielectric constant is a function of the density of the free electrons in this secondary electron resonance. In both cases the losses stem from the kinetic energy lost by the electrons when they collide with neutral particles or the boundaries of their container.

Basically the phase shifter according to the present invention consists of a section of evacuated microwave transmission line and associated elements. These elements allow a multipactor discharge to be sustained within that line at an appropriate density to shift its length by a desired amount. From the description of the multipactor discharge and its interaction with the microwave energy, one can readily see that if control of the electron density of the multipactor discharge can be achieved, then the effective dielectric constant is controlled. If this discharge is occurring in a microwave transmission line, then the propagation constant, hence the phase shift of the region where the discharge occurs, is a variable vwhich can be manipulated.

The control of the multipactor discharge is the heart of the present invention. The numberof free electrons must be defined, and their distribution is important. A discrete boundary between the plasma and the vacuum dielectric will cause reflection or mismatch. Over-concentration of the plasma may cause localized heating and field emission arcs at high-power levels. However, these problems will be reduced because of the natural characteristic of space charge separation as the density of the electrons increases.

The above objects and advantages of the present invention will become apparent upon reference to the following detailed description considered with the accompanying drawings wherein:

FIGURE 1 is an exploded perspective of a first embodiment of a phase shifter according to the invention utilizing magnetic field control;

FIGURE 2 is a cross-sectional view of a second embodiment of the present invention with D.C. bias control; and

FIGURES is a cross-sectional view of a third embodiment of the invention utilizing a DC. bias and grid control.

Referring now to the drawings and FIGURE 1 in particular, a multipactor phase shifter 10 is shown connected to waveguide 12. Phase shifter 10 utilizes a series of electromagnets 14 having pole pieces 16 to produce a transverse magnetic field 18 across a flattened rectangular waveguide section 20. Pole pieces 16 are tapered at ends 22 and 24 to provide a taper of the multipactor discharge to minimize the reflection of the incident microwave energy. Waveguide section 20 is provided with high power microwave windows at each end (not shown) which are transparent. to the microwave energy but which allow the guide to be sealed off for evacuation. To cool the waveguide section 20 it is provided with a water jacket 28. The multipactor discharge occurs in the electrical high electric field region (26) of the guide 20 when a high power microwave signal is propagating through it. There is a range of magnetic field strengths over which multipactor action can be sustained. As the magnetic field 18 is varied through this range the density of the free electrons composing the discharge is varied. As previously explained, this controllable density of free electrons provides an effective dielectric constant that can be manipulated, thus control of the propagation constant and relative electrical phase of the emerging signal is obtained.

Although shown in a rectangular guide with a transverse magnetic field (i.e. transverse to the direction of microwave energy propagation), this same method of control could be used in other transmission lines such as ridged guide, circular guide, TEM lines of rectangular or cylindrical geometry. In addition, the direction of the magnetic field used for control does not necessarily have to be in the transverse direction.

The phase shifter illustrated in FIGURE 2 utilizes a double-ridged waveguide 30 having ridges 32 and 34 therein. The waveguide is divided symmetrically in a plane perpendicular to the RF electric field. This division is accomplished by a conducting septum 36 having several pairs of legs 38 and 38 extending through the sidewalls of the waveguide 30. Septurn 36 is suspended by RF chokes 4t and 40 and ceramic vacuum seals 41 and 41. The RF chokes effectively restrict current flow in septum 36 to DC. only but, nevertheless permit it to remain unobtrusively in the RF circuit. Thus a D.C. bias electric field 42 is impressed between ridges 32 and 34 and septum 36 due to the DC. voltage supply 44 connected between waveguide 30 and septum 36. A single surface D.C. biased secondary electron resonance (multipactor discharge) as described earlier, can be established between the septum (36) and the two ridges (32, 34) when the RF signal is propagating through waveguide 30. There is a range of values of DC. bias voltages over which the multipactor can be sustained. As the DC. supply 44 is varied the electron density of the discharge is changed and the propagation constant of the guide is controlled. The device is cooled by water passageways (46) in each ridge (32, 34).

Other waveguide geometries or TEM lines could be used in a similar manner to the one shown and the DC. bias could conceivably be oriented in a plane other than the one shown parallel to the RF electric field.

Referring now to FIGURE 3, there is illustrated a multipactor phase shifter which uses a grid to control a multipactor discharge in a section of evacuated ridge waveguide.

waveguide section 50 includes a grid 52 forming a broad wall of the waveguide and is dense enough to reduce the RF leakage through it to a negligible amount. Grid 52 is connected electrically and physically as an integral wall of waveguide St). A secondary electron producing surface 54 is closely spaced beneath grid 52 but insulated from it by insulating supports 55. When voltage source 58 causes surface 54 to be biased positive with respect to grid 52, electrons arriving from ridge 56 will fall through grid 52 and release secondary electrons. The departing secondaries that have sufiicient velocity to overcome the positive gradient at their emission point will be accelerated by the much higher RF fields beyond the grid toward ridge 56 where they will impinge and release the next generation of secondaries. The gradient at the surface beneath the grid, determined by the bias voltage, will thus control the density of the multipactor discharge by acting as a velocity filter. From another viewpoint, the grid acts as a screen grid to the arriving electrons and as a control grid to the departing electrons. This embodiment too includes a water passageway 60 for cooling the device.

While this invention has been described with reference to specific embodiments thereof, it will be apparent that various modifications and changes may be made without departing from the spirit of the invention, as defined in the appended claims.

What is claimed is:

1. A multipactor phase shifter for controlling the phase of a microwave signal comprising: a section of evacuated transmission line; a microwave signal source connected to said transmission line for producing a multipactor discharge within said transmission line; and means for controlling the electron density of said multipactor discharge and thereby controlling the propagation constant of said transmission line.

2. A multipactor phase shifter as set forth in claim 1 wherein said transmission line is a rectangular waveguide and said control means comprises an electromagnet for producing a magnetic field within said waveguide.

3. A multipactor phase shifter as set forth in claim 2 wherein said electromagnet includes a pair of pole pieces disposed adjacent the sidewalls of said waveguide for directing the magnetic field across said waveguide, said pole pieces having tapered end portions.

4. A multipactor phase shifter as set forth in claim 1 wherein said transmission line is a double ridged waveguide; and further including a conducting member dividing said waveguide thereby permitting a multipactor discharge adjacent each ridge of said waveguide.

5. A multipactor phase shifter as set forth in claim 4 wherein said control means comprises a DC. control voltage source connected between said dividing member and said waveguide.

6. A multipactor phase shifter as set forth in claim 5 and further including means for limiting current flow in said dividing member to direct current.

7. A multipactor phase shifter as set forth in claim 5 wherein said dividing member includes a portion extending through each side of said waveguide and further including an RF choke surrounding each of said extending portions.

8. A multipactor phase shifter as set forth in claim 1 wherein said transmission line is a single ridged waveguide; and further includes a secondary electron producing surface disposed within said waveguide and insulated therefrom.

9. A multipactor phase shifter as set forth in claim 8 and further including a grid disposed between said ridge and said secondary electron producing surface.

10. A multipactor phase shifter as set forth in claim 8 wherein said control means includes a DC. control voltage connected between said waveguide and said secondary electron producing surface.

References Cited Pringle, D. H.: An Electronic Phase-Shift Tube for Microwave Frequency, Journal Scientific Institute, vol. 32, April 1955, pp. l25l27.

HERMAN KARL SAALBACH, Primary Examiner. L. ALLAHUT, Assistant Examiner. 

1. A MULTIPACTOR PHASE SHIFTER FOR CONTROLLING THE PHASE OF A MICROWAVE SIGNAL COMPRISING: A SECTION OF EVACUATED TRANSMISSION LINE; A MICROWAVE SIGNAL SOURCE CONNECTED TO SAID TRANSMISSION LINE FOR PRODUCING A MULTIPACTOR DISCHARGE WITHIN SAID TRANSMISSION LINE; AND MEANS FOR CONTROLLING THE ELECTRON DENSITY OF SAID MULTIPACTOR DISCHARGE AND THEREBY CONTROLLING THE PROPAGATION CONSTANT OF SAID TRANSMISSION LINE. 